FULL PAPER

Spin-State Dependent Redox Catalytic Activity of a Switchable

Iron(II) Complex

Il’ya A. Gural’skiy,*[ab] Sergii I. Shylin,[ab] Vadim Ksenofontov[a] and Wolfgang Tremel[a]

Abstract: The spin state of catalytically active 3d metal centers plays

a significant role for their activity in enzymatic processes and

organometallic catalysis. Here we report on the catalytic activity of a

Fe(II) coordination compound that can undergo a cooperative switch

between low-spin (LS) and high-spin (HS) states. Catalytic

measurements within 291 – 318 K temperature region reveal a drastic

drop of the catalytic activity upon conversion of metallic centers from

the LS to the HS form. For a thermoswitchable [Fe(NH2trz)3]Br2

complex (Tup = 305 K), an activation energy is found to be

considerably lower for the LS state (158 kJ mol-1) comparing to the

HS state (305 kJ mol-1). Mössbauer analysis reveals that this is related

to a higher conversion of a LS complex upon oxidation. The

comparisons with another polymorph of [Fe(NH2trz)3]Br2 (Tup = 301 K)

and with [Fe(NH2trz)3](ClO4)2 (Tup = 240 K) are made. These results

show the perspective of spin-crossover compounds to compare a

catalytic activity of different spin states within the same material when

other differentiations are minimized.

Introduction

The spin states of metals in transition metal complexes and the

active sites of enzymes are in the focus of (bio) inorganic

chemistry, catalysis and materials science as determinants of

their magnetic properties and chemical reactivity.[1] The

coordination chemistry and the variable oxidation states of

transition metals provide the mechanistic machinery for a

multitude of metal-catalyzed transformations.[2] For reactions

involving paramagnetic intermediates and proceeding to form

radical intermediates it is likely that the spin states of reacting

intermediates (and spin-orbit coupling effects) require

consideration.[3] This is particularly relevant in biological oxidation

catalysis which involves high-valent manganese[4] or iron[5]

intermediates with variable spin states depending on the co-

ligands involved, but also in chemical catalysis,[6] where 3d metals

become increasingly important and have attracted theoretical and

experimental attention.[1]

A typical example are alpha-diimine iron atom transfer radical

polymerization (ATRP) catalysts, where the metal spin state

correlates with the polymerization mechanism.[7] For high-spin

Fe(III) species (S = 5/2), living atom transfer radical

polymerization predominates, whereas for catalysts in an

intermediate spin state (S = 3/2) an organometallic pathway has

led to catalytic chain transfer.[8] It has been shown that spin

transitions between HS and LS states play a key role in β-hydride

elimination reactions of high-spin alkyl complexes. This leads to a

spin-accelerated mechanism with the transition state having a

lower-spin electronic configuration than both reactants and

products.

Metals are required to circumvent spin restrictions imposed for

reactions of triplet oxygen with singlet organic molecules.[9] Thus

Nature uses transition metals in different spin states to practice

catalytic oxidation chemistry on a large scale, examples being

Mn-catalyzed water oxidation to evolve O2[10] or the activation of

carbon hydrogen bonds involving the heme-containing enzymes

cytochrome P450[11] and chloroperoxidase.[12] Here, the electronic

structures and spin states of heme-related Fe-porphyrins are

crucial determinants of their reactivity.[13]

Iron compounds are well known for their catalytic activity with iron

intermediates in high oxidation states.[14] Our approach to probe

the effect of spin state on the catalytic activity was to use Fe(II)

complexes that can exist in both, low-spin (LS) and high-spin (HS)

states depending upon external triggers such as temperature,

pressure or external fields. [15–18] This spin crossover (SCO) effect

resulting from an equilibrium of high- and low-spin states is the

prototype of a switchable molecular solid with applications in

molecular electronics,[19,20] actuating devices,[21,22] displays,[23]

microthermometry,[24,25] and chemical sensing[26] in solid state and

coordination chemistry, biochemistry, geology, and minerology. [27,28]

Results and Discussion

In order to observe the effect of spin state on the catalytic activity

of iron complexes in toluene suspensions we have studied the

redox catalytic activity of tris(µ2-4-amino-1,2,4-triazole)iron(II)

bromide [Fe(NH2trz)3]Br2 (NH2trz = 4-amino-1,2,4-tiazole) 1 – a

one-dimensional coordination polymer built up from iron-triazole

chains with bromide anions situated in the inter-chain

channels.[29,30] Members of the family of Fe(II)-triazole complexes

are known for their spin crossover behavior at or close to ambient

temperature.[31,32] [Fe(NH2trz)3]Br2 displays a hysteretic spin

transition around room temperature. The exact transition

temperature strongly depends on the synthetic procedure. Two

samples of [Fe(NH2trz)3]Br2 prepared from water (1a) or ethanol

(1b) were used for further investigations.

[a] Dr. I.A. Gural’skiy, S.I. Shylin, Dr. V. Ksenofontov, Prof. W. Tremel

Institute of Inorganic and Analytical Chemistry

Johannes Gutenberg University of Mainz

Duesbergweg 10-14, Mainz 55099, Germany

E-mail: illia.guralskyi@univ.kiev.ua

[b] Dr. I.A. Gural’skiy, S.I. Shylin

Department of Chemistry

Taras Shevchenko National University of Kyiv

Volodymyrska St. 64, Kyiv 01601, Ukraine

Supporting information for this article is given via a link at the end of

the document.

FULL PAPER

Figure 1. Temperature-dependent magnetic properties of [Fe(NH2trz)3]Br2 (1a

and 1b) and [Fe(NH2trz)3](ClO4)2 (2) in toluene. 1a and 1b show spin-crossover

behavior between 260 and 320 K, whereas 2 does not change its spin state. A

photograph of the capillary with sample is inserted. Magnetization is given in

arbitrary units. It was not turned in molar units since precise values cannot be

obtained for a very elongated capillary with a liquid diamagnetic toluene inside.

Scheme 1. Model redox reaction to demonstrate the redox catalytic activity of

[Fe(NH2trz)3]Br2.

The spin behavior of the complexes was monitored (in closed

capillaries under liquid toluene, Figure S1) by SQUID

magnetometry (Figure 1). 1a and 1b displayed a cooperative spin

transition around room temperature (1a: Tup = 305 K, Tdown = 296

K; 1b: Tup = 301 K, Tdown = 283 K). These temperatures are similar

to those firstly reported for this complex by L.G. Lavrenova et al.

in 1990 (Tup = 312 K, Tdown = 302 K for the hydrated form; Tup =

302 K, Tdown = 284 K for the dehydrated form).[29] PXRD patterns

of two isomorphic forms obtained by us are given in Figure S2.

For comparison we used the analogous complex

[Fe(NH2trz)3](ClO4)2 (2) containing a perchlorate counter anion

which has no spin transition in the given temperature range

(LS→HS transition occurs at ~240 K, Figure S3). 3,4,5,6-

tetrachlorochatechol (TCC) was used as substrate because its

oxidation product (3,4,5,6-tetrachloro-1,2-chinone, Figure S4)

could easily be detected spectrophotometrically (Scheme 1). The

oxidizing agent was 3-chloroperoxobenzoic acid (CPBA) in

toluene. Nonpolar toluene was specifically used to avoid any

dissolution (solvolysis) of the iron complexes. Tetrachloroquinone

shows a specific adsorption band at 390 nm (Figure 2a) that was

used to monitor the progress of the reaction by UV-Vis

spectroscopy. TCC and CPBA do not absorb at 390 nm (Figure

S5). Without catalyst the reaction is very slow and takes hours,

whereas the catalytic reaction may occur within seconds.

The kinetic curves for this model reaction for different quantities

of 1a in Figure 2a demonstrate a pronounced catalytic efficiency

of the Fe(II)-triazole complex in the solid state, i.e. the reaction

was carried out in a heterogeneous fashion. The reaction was

zeroth order for catechol, whereas the rate depends on the

catalyst concentration. For identical starting concentrations of

catechol and peroxide dA/dt grows linearly with the catalyst

concentration (Figure 2b). For a given precursor concentration

the reaction follows a pseudo-zeroth order kinetics that can be

summarized as

(1)

where ε is the molar extinction of the chinone, l the optical

pathlength, and keff the effective rate constant.

To demonstrate the SCO effect on the catalytic activity of 1a, we

studied the temperature dependent kinetics for the reaction

between 18 and 45 °C. The corresponding kinetic curves are

displayed in Figure 3a. Conversion after 1 min in the reactions

carried out at different temperatures are shown in Figure 3b. All

reactions are pseudo-zeroth order in catechol irrespective of the

temperature.

Figure 2. Catalytic effect of [Fe(NH2trz)3]Br2 on the kinetics of the TCC oxidation

at 39 °C. (a) Kinetic curves for different concentrations of 1a. The insert shows

the evolution of the UV-Vis spectrum upon catechol conversion. (b) Conversion

rate as a function of the catalyst concentration.

eff

chinonechinone kdtl

dA

dt

dc

)(

FULL PAPER

Figure 3. Effect of a spin state on the catalytic activity in the reaction of TCC oxidation (cTCC = 0.309 mM, CCPBA = 4.48 mM, Ccomplex = 2.14 mM). (a) Kinetic curves

of TCC oxidation catalyzed by 1a at different temperatures within 18-45 °C region. (b) Yield of tetrachlorochinone after 1 min of reactions conducted at different

temperatures. (c) Dependence of the reaction rate constant on temperature. (d) Arrhenius plot demonstrates the presence of two kinetic regimes for different spin

states of the catalyst.

Final concentration of tetrachloroquinone (concentration on plato)

is the same for all reactions made at different temperatures (~

0.32–0.36 mmol L-1). This compound can be considered as a

principal oxidation product (intermediate) which is stable in the

timeframes of these reactions, that is why its concentration was

monitored here. It should be noted that this quinone may undergo

further condensation and dechlorination transformations. These

following transformations of tetrachloroquinone are responsible

for the gradual decrease of its concentration after reaching the

plato, with a pronounceable slope observed at elevated

temperatures (see 45°C curve in Figure 3a).

The rate constants keff vs. temperature (extracted from the

gradients) are shown in Figure 3c, and the temperature-

dependence of rate constants according to

(2)

is given in Figure 3d, with A as pre-exponential factor, R the

universal gas constant, and Ea as activation energy.

The reaction rate shows Arrhenius behavior and gradually

increases with temperature between 18 and 36 °C. A drastic drop

of the catalytic activity at 36 °C is associated with the SCO.

After the change of spin state was complete, the temperature

dependence between 39°C and 45 °C showed Arrhenius behavior

again. In essence, Figure 3 clearly differentiates two distinct

temperature regions associated with the FeII(HS) and FeII(LS)

states of 1a. Using Equation 2 the activation energies for the spin

states could be extracted as EaLS = 158(11) kJ mol-1 and Ea

HS =

305 kJ mol-1. This abrupt increase corroborates with the

observation that the LS state was more active than the HS state.

Its origin is assumed to be related to the electronic structures of

two forms and may be due to differences in the electronic

multiplicity, redox potential, lattice energies, etc.

To confirm the effect of SCO on this redox reaction, we carried

out analogous experiments with 1b ([Fe(NH2trz)3]Br2 (precipitated

from ethanol rather than from water) that displayed a lower

transition temperature than 1a (Figure 1). Rate constants as a

function of temperature are shown in Figure 4a. The most

important conclusion is that the effect of the spin transition on this

reaction was observed as well. The reaction rate followed an

Arrhenius behavior between 23°C and 29 °C and showed a

pronounced drop after the SCO.

TR

EAk a

eff

1)ln()ln(

FULL PAPER

Figure 4. Temperature-dependent catalytic activity of Fe(II)-triazolic complexes (cTCC = 0.309 mM, CCPBA = 4.48 mM, C1b = 2.1 mM, C2 = 2 mM). (a) Rate constants

of TCC oxidation catalyzed by 1b at different temperatures within 23-42 °C. (b) Rate constants of TCC oxidation catalyzed by 2 at different temperatures within 26-

47 °C. (c) Arrhenius plot for the oxidation catalyzed by 1b demonstrates an effect of spin state on the catalytic activity. (d) Arrhenius plot for the oxidation catalyzed

by a HS complex 2 demonstrates a classical dependence of the reaction rate on temperature.

The simultaneous presence of the FeII(HS) and FeII(LS) states in

the spin equilibrium range around 32°C leads to a superposition

of the reactivities of both states in Figure 3 (for 1a) and Figure 4

(for 1b). After the transition was complete for 1b at 36 °C the

reaction rate increased again in an Arrhenius-type manner. This

sudden change in the reaction rate agrees with the transition

temperature of 1b (a systematic shift related to the thermalization

in kinetic experiments should be considered when comparing with

magnetic measurements). The slight difference in the

temperature of the “activity drop” for 1a and 1b is assumed to be

related to the different transition temperatures.

Even more informative is the comparison with a sample that

displays no spin transition. We have synthetized 2 that contains

iron-triazole chains as 1a and 1b, but the chains are separated by

perchlorate rather than bromide anions. As a result, 2 adopts the

HS state in toluene suspension above room temperature (Figure

1).

The activation energies of EaLS = 325(41) kJ mol-1 and Ea

HS =

504(46) kJ mol-1 derived for the LS and HS states 1b (Figure 4c)

are slightly higher than those for 1a; comparison of HS and LS

activation energies indicates that the LS state is more reactive

than the HS state. Complex 2 showed Arrhenius behavior (Figure

4d) without deviations from linearity. The activation energy EaHS =

74(2) kJ mol-1 was small and lower than that observed for the LS

and HS states of 1a and 1b.

To understand this difference in the catalytic activity, we have

analyzed the tendency of the two spin states to be oxidized,

because iron in high valence states is assumed to play a crucial

role as intermediate. Mössbauer spectroscopy is the most

convenient way to monitor the relative amounts of iron in different

oxidation and spin states in solid samples.[33] Mössbauer spectra

of 1a and 2 have hyperfine parameters very well corroborating

with those obtained by Lavrenova et al.[29] We recorded

Mössbauer spectra of 1a and 2 that were treated with an excess

of CPBA to achieve a partial (but considerable) oxidation of 1a

and 2 (named 1a-ox and 2-ox). Moreover, a reduction by TCC

was used to reduce iron to its initial form (1a-ox-red and 2-ox-

red) and thus reproduce a whole catalytic cycle. Mössbauer

spectra of precursors and oxidized samples are shown in Figure

5. Each oxidized sample contains iron in Fe(II) and Fe(III)

oxidation states (oxidized form can be different within a catalytic

cycle, e.g. Fe(IV) frequently plays a role of an intermediate). Their

principal hyperfine parameters are summarized in Table 1.

Oxidation does not affect the spin state of Fe(II) centers. In its

order, the oxidation state of newly formed Fe(III) centers is hardly

definable just from hyperfine parameters.[34] What is also

important, TCC can majorly reduce these Fe(III) fractions back to

initial Fe(II) forms (samples 1a-ox-red and 2-ox-red) as shown in

Figure 5. Complex 1a used for the kinetic measurements does

not show any considerable content of the oxidized form after one

catalytic cycle (Figure S8).

FULL PAPER

Table 1. Mössbauer hyperfine parameters of 1a, 2, 1a-ox and 2-ox. A higher conversion to FeIII is displayed by a HS complex comparing to a LS complex (cTCC

= 300 mM, CCPBA = 130 mM, Ccomplex = 20 mM).

Reduced form (FeII) Oxidized form (FeIII)

Content (%) Spin state δ

(mm s-1)

ΔEQ

(mm s-1)

Content (%) δ

(mm s-1)

ΔEQ

(mm s-1)

[Fe(NH2trz)3]Br2 (1a) 100 LS 0.436(3) 0.207(7)

[Fe(NH2trz)3](ClO4)2 (2) 100 HS 1.041(2) 2.781(5)

1a-ox 25(3) LS 0.37(1) 0.2(fixed) 75(3) 0.37(1) 0.78(2)

2-ox 71.0 (8) HS 1.04(1) 2.78(2) 29.0(8) 0.22(1) 0.73(2)

1a-ox-red 82(3) LS 0.40(1) 0.21(2) 18(5) 0.31(2) 0.79(3)

2-ox-red 85(1) HS 1.044(1) 2.818(3) 15(2) 0.19(4) 0.77(6)

Figure 5. Mössbauer spectra of 1a and 2, their oxidized (1a-ox and 2-ox) and

newly reduced (1a-ox-red and 2-ox-red) forms. Fe(II) species are shown in

gray and Fe(III) are shown in black. The content of the Fe(III) species reflects a

higher conversion of LS Fe(II) (vs. HS Fe(II)) upon oxidation.

When using model peroxidase substrates instead of TCC, e.g.

3,3’,5,5’-tetramethylbenzidine or 2,2’-azino-bis(3-

ethylbenzothiazoline-6-sulphonic acid), triazolic complexes do not

display any catalytic activity that excludes a peroxidase-like

mechanism. We believe that the studied catalytic cycle has two

principal steps: (i) oxidation of the iron complex by peroxoacid and

(i) reduction of the complex by TCC. As revealed by Mössbauer

measurements, both of these steps can be reproduced separately.

The higher catalytic activity of the LS species can reasonably be

referred to their higher inclination towards oxidation under

otherwise identical conditions.

Both complexes (1a and 2) undergo a partial oxidation, while the

Fe(III) content is different in each case. When the 1a in its LS state

(at room temperature) is oxidized, the ferric form is present in

amounts of 75(3) %. However, when 2 reacts in its HS state (at

room temperature), the Fe(III) content is just 29(1) %. This

considerable difference reveals that for very similar complexes,

the spin state plays a prominent role in the oxidation process. This

observation is in line with the results of the catalytic oxidation of

TCC.

Conclusions

We have found that in the SCO compound [Fe(NH2trz)3]Br2 the

spin state of the Fe(II) centers plays a crucial role in determining

their redox catalytic activity. The LS species are characterized by

higher reaction rates and smaller activation energies compared to

the HS analogues. We demonstrated that this difference is driven

by a higher tendency of LS iron(II) to be oxidized. Such spin-

dependent activity will be analyzed for other switchable SCO

complexes and other types of chemical reactions in order to

derive a broader picture of the effect of spin state on the catalytic

metal centers, in particular as photochemical and electrochemical

activities may be very sensitive to the spin state of the catalyst.

Application of chiral switchable complexes[35,36] may lead to a

switchable stereoselectivity in catalysis.

FULL PAPER

Experimental Section

Synthesis. [Fe(NH2trz)3]Br2 (1a). NH2trz (250 mg, 2.98 mmol) in water (1

mL) and FeBr2 (210 mg, 0.97 mmol)) in water (1 mL) were mixed under

stirring. The precipitate was formed within 1 h and separated by

centrifugation (13000 rpm, 4 min), washed with water and dried in air. Yield

is 344 mg (74 %). Anal. Calcd for C6H12N12Br2Fe: C, 15.40; N, 35.92; H,

2.59. Found: C, 15.75; N, 36.12; H, 2.31.[Fe(NH2trz)3]Br2 (1b). The sample

was obtained as described for 1a, by replacing water with ethanol in all

steps. Yield is 382 mg (82 %). Anal. Calcd for C6H12N12Br2Fe: C, 15.40; N,

35.92; H, 2.59. Found: C, 15.51; N, 35.96; H, 2.48.

[Fe(NH2trz)3](ClO4)2 (2). The sample was prepared as 1a starting from

NH2trz (250 mg, 2.98 mmol) and Fe(ClO4)2·6H2O (360 mg, 0.99 mmol).

Yield is 330 mg (65 %). Elemental analysis was not performed for safety

reasons.

Catalysis. Catalytic measurements were performed in 3.5 mL quartz

cuvettes with 1 cm optical pathway. The temperature in the cuvette holder

was controlled with a Haake C50P thermostat. The concentration of the

product was monitored by measuring the absorption of quinone with a UV-

Vis Carry spectrometer at 390 ± 2.5 nm with a 5 sec interval.

A catalyst (3 mg) was added to the TCC (0.23 mg) in toluene (2.9 mL). The

cuvette was thermalized for 4 min prior to each measurement. CPBA in

toluene (0.1 mL, 0.13 mol L-1) was added after and a UV-Vis monitoring

started directly. No stirring during the reaction was applied.

Oxidized samples. To produce 1a-ox and 2-ox, a Fe(II) complex (0.1

mmol) was mixed with 3-chloroperoxobenzoic acid in toluene (3 ml, 0.13

mol L-1) and allowed to stay for 5 min at 20 °C. To produce 1a-ox-red and

2-ox-red, oxidized samples (0.1 mmol) were treated with TCC solution (3

ml, 0.3 mol L-1) and allowed to stay for 5 min at 20 °C. The powders were

separated from solutions by centrifugation (13000 rpm, 1 min), washed

with toluene and dried in air.

Magnetic susceptibility measurements. Temperature-dependent

magnetic susceptibility measurements were carried out with a Quantum-

Design MPMS-XL-5 SQUID magnetometer with a heating and cooling rate

of 1 K min−1, and a magnetic field of 0.5 T. A powder sample mixed with

liquid toluene was sealed using a hydrogen torch in a glass capillary (~ 4.5

mm long, inner diameter is 1.0 mm, outer diameter is 1.4 mm). Capillary

was fixed between two gelatin capsules. Molar magnetization was not

calculated because of the considerable diamagnetic impact of toluene and

of the capillary, which could not be accurately subtracted.

Mössbauer spectroscopy. 57Fe-Mössbauer spectra were recorded in

transmission geometry with a 57Co source in a rhodium matrix using a

conventional constant-acceleration Mössbauer spectrometer. Isomer

shifts are given with respect to an α-Fe foil at ambient temperature. Fits of

the experimental Mössbauer data were performed using the Recoil

software (Lagarec and Rancourt, Ottawa University).

Acknowledgements

This work was financed by H2020-MSCA-IF-2014 grant 659614.

We acknowledge useful commentaries from Prof. I.O. Fritsky,

spectroscopic measurements from D. Spetter and graphical

contribution from O.I. Kucheriv. We also acknowledge very useful

comments from the anonymous reviewer on the mechanism of

the studied reaction.

Keywords: iron • spin crossover • spin state • catalysis •

Mössbauer spectroscopy

[1] M. Swart, M. Costas, Eds. , Spin States in Biochemistry and Inorganic

Chemistry, John Wiley & Sons, Ltd, Oxford, UK, 2015.

[2] R. H. Holm, P. Kennepohl, E. I. Solomon, Chem. Rev. 1996, 96, 2239–

2314.

[3] T. Risse, D. Hollmann, A. Brückner, in Catalysis 2015, 27, 1–32.

[4] T. Yang, M. G. Quesne, H. M. Neu, F. G. Cantú Reinhard, D. P. Goldberg,

S. P. de Visser, J. Am. Chem. Soc. 2016, 10.1021/jacs.6b05027.

[5] M. Higuchi, Y. Hitomi, H. Minami, T. Tanaka, T. Funabiki, Inorg. Chem.

2005, 44, 8810–8821.

[6] G. Xue, R. De Hont, E. Münck, L. Que, Nat. Chem. 2010, 2, 400–405.

[7] L. E. N. Allan, M. P. Shaver, A. J. P. White, V. C. Gibson, Inorg. Chem.

2007, 46, 8963–70.

[8] M. P. Shaver, L. E. N. Allan, H. S. Rzepa, V. C. Gibson, Angew. Chemie

Int. Ed. 2006, 45, 1241–1244.

[9] J. S. Valentine, Bioinorg. Chem. 1994, 313–523.

[10] V. Krewald, M. Retegan, F. Neese, W. Lubitz, D. A. Pantazis, N. Cox,

Inorg. Chem. 2016, 55, 488–501.

[11] F. P. Guengerich, A. W. Munro, J. Biol. Chem. 2013, 288, 17065–17073.

[12] M. T. Green, J. Am. Chem. Soc. 2006, 128, 1902–1906.

[13] M. E. Ali, B. Sanyal, P. M. Oppeneer, J. Phys. Chem. B 2012, 116, 5849–

5859.

[14] A. Ghosh, D. Mitchell, A. Chanda, A. D. Ryabov, D. L. Popescu, E. C.

Upham, G. J. Collins, T. J. Collins, V. Pennsyl, J. Am. Chem. Soc. 2008,

130, 15116–15126.

[15] S. Brooker, Chem. Soc. Rev. 2015, 44, 2880–2892.

[16] M. A. Halcrow, Spin-Crossover Materials, John Wiley & Sons Ltd, Oxford,

UK, 2013.

[17] P. Gütlich, Eur. J. Inorg. Chem. 2013, 2013, 581–591.

18] P. Gütlich, H. A. Goodwin, in Top. Curr. Chem., Springer, 2004.

[19] J. Dugay, M. Giménez-Marqués, T. Kozlova, H. W. Zandbergen, E.

Coronado, H. S. J. van der Zant, Adv. Mater. 2015, 27, 1288–1293.

[20] A. Rotaru, J. Dugay, R. P. Tan, I. A. Guralskiy, L. Salmon, P. Demont, J.

Carrey, G. Molnár, M. Respaud, A. Bousseksou, Adv. Mater. 2013, 25,

1745–1749.

[21] H. J. Shepherd, I. A. Gural’skiy, C. M. Quintero, S. Tricard, L. Salmon, G.

Molnár, A. Bousseksou, Nat. Commun. 2013, 4, 2607.

[22] I. A. Gural’skiy, C. M. Quintero, J. S. Costa, P. Demont, G. Molnár, L.

Salmon, H. J. Shepherd, A. Bousseksou, J. Mater. Chem. C 2014, 2,

2949–2955.

[23] O. Kahn, J. Kröber, C. Jay, Adv. Mater. 1992, 4, 718–728.

[24] L. Salmon, G. Molnar, D. Zitouni, C. Quintero, C. Bergaud, J.-C. Micheau,

A. Bousseksou, J. Mater. Chem. 2010, 20, 5499–5503.

[25] I. A. Gural’skiy, C. M. Quintero, K. Abdul-Kader, M. Lopes, C. Bartual-

Murgui, L. Salmon, P. Zhao, G. Molnar, D. Astruc, A. Bousseksou, J.

Nanophotonics 2012, 6, 63513–63517.

[26] J. A. Real, A. B. Gaspar, M. C. Muñoz, Dalt. Trans. 2005, 2062.

[27] B. Chance, Science 1968, 159, 654–658.

[28] T. Kawakami, Y. Tsujimoto, H. Kageyama, X.-Q. Chen, C. L. Fu, C.

Tassel, A. Kitada, S. Suto, K. Hirama, Y. Sekiya, Y. Makino, T. Okada,

T. Yagi, N. Hayashi, K. Yoshimura, S. Nasu, R. Podloucky, M. Takano,

Nat. Chem. 2009, 1, 371–376.

[29] L. G. Lavrenova, V. N. Ikorskii, V. A. Varnek, I. M. Oglezneva, S.V.

Larionov, Koord. Khim. 1990, 16, 654–661.

[30] O. Fouché, J. Degert, G. Jonusauskas, N. Daro, J.-F. Létard and E.

Freysz, Phys. Chem. Chem. Phys. 2010, 12, 3044–3052.

[31] L. G. Lavrenova, O. G. Shakirova, Eur. J. Inorg. Chem. 2013, 2013, 670–

682.

[32] O. Roubeau, Chem. Eur. J. 2012, 18, 15230–15244.

[33] P. Gütlich, Y. Garcia, in Mössbauer Spectrosc. (Eds.: Y. Yoshida, G.

Langouche), Springer Berlin Heidelberg, Berlin, Heidelberg, 2013, pp.

23–89.

[34] F.Neese, Inorg. Chim. Acta 2002, 337, 181–192.

[35] I. A. Gural’skiy, O. I. Kucheriv, S. I. Shylin, V. Ksenofontov, R. A. Polunin,

I. O. Fritsky, Chem. Eur. J. 2015, 21, 18076–18079.

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[36] I. A. Gural’skiy, V. A. Reshetnikov, A. Szebesczyk, E. Gumienna-

Kontecka, A. I. Marynin, S. I. Shylin, V. Ksenofontov, I. O. Fritsky, J.

Mater. Chem. C 2015, 3, 4737–4741.

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Entry for the Table of Contents

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Spin-crossover Fe(II) complex

[Fe(NH2trz)3]Br2 shows a redox

catalytic activity that changes upon

conversion of metallic centers from

the low-spin to the high-spin form.

The activation energy is

considerably smaller for the low-

spin state. This difference is

related to a higher tendency of the

diamagnetic form towards

oxidation.

Il’ya A. Gural’skiy,* Sergii I.

Shylin, Vadim Ksenofontov and

Wolfgang Tremel

Page No. – Page No.

Spin-State Dependent Redox

Catalytic Activity of a

Switchable Iron(II) Complex